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I will at this point revert to Posidonius’ opinion of the cause of thunder. From the earth and its confines are exhaled certain elements, partly moist, partly dry and smoke-like. The latter element remains in the sky as material for lightning, while the former falls in rain. The dry smoky particles that reach the atmosphere will not allow themselves to be enclosed in clouds, but burst their envelope. Thence comes the report which we name thunder. Besides this, anything in the atmosphere itself that is rarefied is at the same time dried and heated up. This also, if it is enclosed, seeks an exit with equal eagerness, and causes a report as it escapes. On one occasion it makes a complete burst, and the thunder is consequently the more violent; on another it escapes by degrees in small portions. Air of this kind, then, by either bursting or flying through the clouds, produces peals of thunder. The rolling of the air enclosed in a cloud is the most potent cause of setting fire to what is struck.